Basically any upbeat song that reminds you to appreciate life in the middle of absorbing and applying theory. Anything from the Internationale to Red Army tunes to modern(ish) artists like Chumbawamba are allowed. Even Lofi.
Personal favorite of mine is Quand Fera-T-Il Jour, Camarade? by Mireille Mathieu. It's bittersweet, hopeful and resolute. Even if I don't speak French, I sometimes mumble to it.
Runner-up would be Solidarity Forever by Pete Seeger.
I think this one fits: Apesar de você , by Chico Buarque
It's a very hopeful song composed at the height of Brazil's military dictatorship, and one of my all time favorites. The title means "In Spite of You", and the lyrics are about communion and the innevitability of the better times that (were) are bound to come, with just a dash of revenge for flavor